GPR, or ground-penetrating RADAR (where RADAR is “RAdio Detection and Ranging”), is a technology used to assess the composition and location of heterogeneous materials. GPR uses common radio frequencies and is particularly useful in that it is both non-destructive and non-ionizing. In fact, GPR uses frequencies similar to those of a cellular phone, but at far lower power levels. Common applications include locating the precise position of rebar within a concrete wall/floor, identifying and locating buried objects underground, assessing the quality and uniformity of an asphalt or concrete highway surface, and detecting deterioration on bridge decks. In road surface applications, GPR is used, for example, to detect cracks, fissures, or contamination in any one of the chip seal, pavement layers, gravel base, and so forth. In many roadway applications, a resolution of features of the road surface of less than one inch (2.54 cm) is desired. Such systems may be mounted on vehicles, traveling over the surface while acquiring measurement data. GPR systems are disclosed in more detail in U.S. Pat. No. 5,499,029 to Bashforth, et al., and U.S. Pat. No. 5,384,715 to Lytton, which are hereby incorporated by reference.
FIG. 1 shows a sub-sampled GPR System which acquires one sample of the reflection waveform per transmitted cycle by way of a fast sampler and a variable delay line, as known in the art. The value of the variable delay line 19 advances for each transmitted cycle until the entire time range is captured. Traditional impulse GPR systems use a sub-sampling scheme which achieves high resolution measurements at low cost, such as shown in FIG. 1, but at the expense of being sub-optimally noisy. Each time the transmitter 12 sends a pulse (such as incident transmission 10), the receiver 14 detects the response 15 (the reflected waveform of a measurement object 11) only at a very precise instant of time which is advanced in position with each successive transmission. Such a technique is possible because the RADAR analog waveform can be reproduced as many times as necessary, such that it is not necessary to capture all of the information at once. This technique is generally referred to as equivalent time sampling (ETS). Hundreds or even thousands of pulses are transmitted in order obtain a full measurement set over the desired time range. A further refinement of this technique is to direct more sampling effort to portions of the waveform where additional SNR enhancement is desired. However, most of the received energy is discarded (the energy present at times other than the discrete sampling point) and the resultant measurement is inefficient, in terms of noise, relative to the amount of energy that was transmitted (see FIG. 2).
FIG. 2 shows an example of a timing diagram used in the prior art to carry out GPR measurements. The figure shows how one point (21, 22, 23, 24, 25, and 26, where 26 shows multiple points) is acquired from each reflected RADAR waveform (that is, Responses 1 through 5 and Response N, as hundreds or thousands of data points may actually be used) and compiled to form one single time range measurement. A noise-free, optimal result is shown as noise-free graph 28. Observe that the compiled result (the subsampled response 29) has high frequency noise, while the constituent individual RADAR responses contain slowly varying noise.
Recovery of all or most of the reflected RADAR information greatly improves the measurement signal energy with respect to noise; more efficient signal detection directly translates to improved system performance. This improved detection technique is referred to as High Speed Interpolated Sampling. Until recently, real-time sampling techniques for most GPR applications have been prohibitively expensive and required unreasonable levels of power consumption because of the difficulty in sampling and quantizing an analog waveform at such high speeds. However, advances in integrated circuit technology have enabled the development of nearly-suitable low-cost monolithic devices that perform at reasonably high speeds while consuming relatively small amounts of power.
Therefore, based on a need in the art to find a better way to resolve weak targets when employing ground-penetrating radar at high speeds, the present technology improves upon GPR measurement techniques as follows.